1. Field of the Invention
The present invention relates to a photopolymerizable coating composition containing a color pigment, which is ultraviolet curable and which is capable of providing a coating layer excellent in the abrasion resistance and water resistance, and a process for forming a coating of a stereoscopic pattern by using such a composition.
2. Discussion of Background
In recent years, photopolymerizable coating compositions containing no solvent or little organic solvent have been employed in view of social demands for conservation of resources and pollution free operation. However, conventional photopolymerizable coating compositions are mainly of a clear type containing no color pigment. An enamel type composition containing a color pigment is practically used only in a limited area since the coating thickness is limited, and its commercial development is stalemated.
As mentioned above, a photopolymerizable coating composition containing a color pigment has a difficulty in that when coated in a substantial thickness, ultraviolet rays will not sufficiently be transmitted to the interior of the coating layer, whereby inadequate curing will result, and wrinkles are likely to form with the curing of only the surface of the layer, thus leading to a poor outer appearance of the coating layer Further, the coating efficiency used to be poor.
Under these circumstances, it has been proposed to use as the color pigment a pigment which hardly absorbs ultraviolet rays, or to use a highly reactive photopolymerizable compound, or to use as the ultraviolet lamp a lamp capable of emitting light rays of long wavelengths. However, in either proposal, the coating thickness is limited to a level of at most 30 .mu.m. If the thickness exceeds this limit, defective curing tends to result, and the coloring power and the opacifying power tend to be poor as compared with usual enamels.
Further, it has been recently proposed to incorporate spherical glass particles to a photopolymerizable coating composition containing a color pigment (e.g. Japanese Unexamined Patent Publication No. 79275/1987). By this method, it is possible to obtain a coating layer having the curability improved to some extent. However, the composition is inferior in the water resistance and exhibits poor curability by light rays with short wavelengths.
On the other hand, as photoinitiators to be used for photopolymerizable coating compositions, benzophenone, acetophenone, diethoxyacetophenone, chloroacetophenone, 2,2-dimethoxy-2-phenylacetophonone, propiophenone, thioxanthone, benzoin, benzyl, anthraquinone, benzylketal, benzoin ethyl ether, benzoin isopropyl ether and dimethoxyphenyl are known as typical examples (U.S. patent application Ser. Nos. 129,297, 731,888 and 757,249). However, photoinitiators commonly employed including those mentioned above, are incapable of sufficiently curing the coating layers to the inside thereof if the photopolymerizable coating compositions containing them contain a substantial amount of an opacifying component such as a color pigment.
Further, acylphosphine oxide compounds have been developed as photoinitiators (U.S. Pat. Nos. 4,292,152 and 4,385,109). These compounds are excellent in the photosensitivity as compared with the above-mentioned photoinitiators and capable of forming cured coating layers even when used for photopolymerizable coating compositions containing color pigments. However, the coating thickness is still limited to a level of at most a few tens .mu.m. If the coating thickness is at a level of a few hundreds .mu.m, the inside of the coating layer will not sufficiently be cured. Further, even in a case of a thin coating layer, if a large amount of a color pigment is incorporated to increase the opacifying power, the curability tends to be poor, and the resulting coating layer tends to be inferior in the water resistance, etc.
In the fields of interior decorating materials or building materials, it has been common to employ materials having stereoscopic colorful patterns applied to present a beautiful outer appearance. Such stereoscopic colorful patterns are usually applied by forming multiple coating layers of different colors by (i) a gravure, offset or offset gravure printing method, (ii) a screen printing method or (iii) a coating method, or by a combination of these methods. However, it is common to use an organic solvent for the coating material or for an ink used for such methods.
On the other hand, in view of the social demands for pollution free operation and conservation of resources, there has been in recent years a tendency of adopting a photopolymerizable coating composition containing no solvent or a less amount of an organic solvent. Therefore, it is conceivable to use a photopolymerizable coating composition containing no or little organic solvent, as the coating composition for forming such a stereoscopic colorful pattern. However, when such a photopolymerizable coating composition is to be used as a coating composition for forming a stereoscopic colorful pattern by multiple coating layers, the coating composition is required to satisfy the following conditions:
(1) It provides good intercoat adhesion when multiple coating layers are formed.
(2) It provides consistent curability within a wide light energy range.
(3) It provides constant curability even when the coating layer thickness varies.
(4) It is capable of maintaining the initial color of coating, which is not affected even when irradiated several times with ultraviolet rays.
However, none of the conventional photopolymerizable coating compositions fully satisfies these conditions. Therefore, stereoscopic colorful patterns formed by colored photopolymerizable coating compositions have not substantially practically used.
Namely, when a stereoscopic colorful pattern is formed by multiple coating layers, the coating layer formed first i.e. the undercoating layer, is exposed several times to ultraviolet rays and accordingly will be in an overcured state, whereby it is likely that the intercoat adhesion deteriorates or the layer undergoes a color change.
Further, the light emitting element of the ultraviolet curing apparatus is usually a tubular lamp, and the intensity of lights of the lamp in the parallel direction is not constant. Particularly, the intensity of lights is low in the vicinity of the electrodes. Further, the light intensity tends to decrease by the deterioration with time of the lamp. Under these conditions, the coating layer is likely to have a partial curing defect, and a color change is likely to take place. Further, in the case of ultraviolet curing, ultraviolet rays enter from the surface of the coating layer and reach to the boundary of the coating layer with the substrate to complete the curing reaction However, it is usual that the thickness of the coating layer varies depending upon the nature of the coating composition such as the viscosity, the coating or printing conditions or the surface conditions of the substrate such as the surface roughness of the substrate or the penetrability of the coating composition. Consequently, it is likely that there will be some portions deep in the coating layer where the curing reaction is partially inadequate, and adhesion defects, blisters or wrinkles are likely to result.